When a silver halide photographic light-sensitive material is exposed to light to which it is sensitive and developed, the black density of the light-sensitive material generally increases with the increase of exposure and reaches a maximum value at a certain exposure, and a further increase in exposure decreases the black density. This phenomenon is called solarization.
Therefore, when an appropriate fog is optically or chemically given beforehand to a silver halide emulsion so as to yield a maximum black density, solarization is caused by exposure, making a positive image directly. A light-sensitive material utilizing such a reversal phenomenon is called a direct positive silver halide photographic light-sensitive material of latent image destruction type (hereinafter referred to as a direct positive light-sensitive material).
Direct positive light-sensitive materials of this type are used, for example, in copying photographs of various types.
Conventional direct positive light-sensitive materials of this type have some problems to be solved. One of the problems is a difficulty in imparting an appropriate high sensitivity to these light-sensitive materials. That is, conventional light-sensitive materials of this type mostly use a silver halide emulsion having a wide grain size distribution, and every silver halide grain does not undergo an optimum chemical sensitization constantly, thereby the inherent sensitivity of these silver halide grains is not brought out fully.
Another problem arises from an increasing demand for rapid processing in recent years. That is, with the rapid expansion of silver halide photographic light-sensitive material consumption, the number of light-sensitive materials to be developed increases markedly. Accordingly, more rapid processing, or more increase in processing volume in a fixed period of time, has come to be strongly demanded. This tendency is seen in various end uses of light-sensitive materials. In light-sensitive materials for X-ray photography, for example, strict enforcement of periodical medical checks leads to a rapid increase in the number of diagnoses, and more accurate diagnoses lead to increase in the number of checking items, which results in more and more increase in a radiographing frequency and necessitates a more rapid processing.
A more rapid processing is strongly demanded of direct positive light-sensitive materials, too; therefore, it is necessary to impart a rapid processability to these light-sensitive materials, in addition to the automatization of diagnoses (in radiographing, conveying, etc.).
However, direct positive light-sensitive materials have a disadvantage that they tend to lower the sensitivity when subjected to a rapid processing in which the overall processing time is not less than 20 seconds and not more than 60 seconds.
The lowering in the sensitivity can be prevented by increasing the coating amount of silver halide emulsion, but this generates the following adverse effects, excluding a rise in film cost as a natural consequence.
(a) insufficient fixation, PA0 (b) insufficient film washing, and PA0 (c) insufficient film drying.
Therefore, in imparting a rapid processability to the direct positive light-sensitive material, it is necessary to develop a technique which does not lower the sensitivity and maximum density even when the coating amount of silver halide is decreased. Examples of the techniques to impart a high sensitivity with a reduced amount of silver halide can be seen in U.S. Pat. Nos. 2,996,382 and 2,178,382, in which silver halide grains of surface latent image type are allowed to adjoin silver halide grains having fogged specks inside of the grains in order to provide a light-sensitive material having a high sensitivity, sharp contrast and high covering power.
However, these techniques alone cannot impart an appropriately high maximum density and appropriately low minimum density constantly, and preventives against such a problem are apt to deteriorate the sensitivity and contrast.
That is, problems such as a high minimum density (fog) and poor graininess arise when said light-sensitive materials are subjected to a rapid processing in an automatic developing machine whose overall processing time (including crossover time) is not less than 60 seconds and not more than 120 seconds. Addition of various additives to an emulsion or a developer is known as a preventive measure against such a low minimum density, but any of the additives more or less deteriorates the sensitivity, contrast and graininess. Use of gelatin in an increased amount is known as a means to improve the graininess, but it has a defect to lower the sensitivity, gamma and maximum density.
As described above, conventional techniques cannot provide fully satisfactory photographic characteristics including appropriate maximum and minimum densities, a high sensitivity and an sufficient contrast.